Recently, technical developments for attaining a high fuel economy of automobiles have been conducted actively from the standpoint of diminishing the environmental load. AS to the valve spring and suspension spring which are automobile parts, studies are being made about an increase of design stress and the reduction of size. In this connection, the spring steel used is required to have a high strength. Generally, however, when metallic materials are rendered high in strength, their brittle fracture resistance typified by fatigue and delayed fracture is deteriorated. Therefore, for attaining a high strength, it is required to make it compatible with the resistance to fracture.
To meet such a requirement, for example in Japanese Patent Laid-open (JP-A) No. 06-306542 there is proposed a spring steel improved in fatigue strength by controlling the composition of a non-metallic inclusion and in JP-A No. 10-121201 there is proposed a high strength spring steel improved in the resistance to delayed fracture by controlling the amount of P segregation in the pre-austenite grain boundary of steel having the structure of martensite. Further, in JP-A No. 2003-306747 is proposed a spring steel improved in the resistance to fatigue by controlling the residual γ, in JP-A No. 2003-213372 is proposed a spring steel improved in the resistance to fatigue by controlling the pre-austenite grain size. In JP-A No. 2003-105485 is disclosed a high strength spring steel improved in the resistance to hydrogen-induced fatigue fracture by making the steel structure into a lamellar structure of martensite and ferrite.